1. Field of the Invention
This invention relates to the field of error code correction of recorded digital signals to correct for noise bursts and signal dropouts typical of signal recording and playback systems. In particular, the invention relates to the implementation of code correction techniques to singletrack recording formats such as are used in video tape recorders.
2. The Prior Art
Although this invention is useful in minimizing errors that are produced by various types of signal processing apparatus, it is particularly useful in minimizing errors in processing pulse code modulated (PCM) audio signals by a video tape recorder (VTR).
The frequency response of VTR amplifiers and transducers, even those intended for home use, is much better than is required for direct recording of audio signals, but VTRs are not directly suited for handling non-repetitive, analog signals, such as audio signals.
VTRs include rotary head means and synchronizing circuits for recording and playing back television signals, which are divided into fixed time segments by horizontal and vertical synchronizing signals. The necessity for providing synchronizing circuits and for including synchronizing signals with the signals being processed makes direct use of VTRs in recording audio signals difficult. Inserting synchronizing pulses on top of analog audio signals or substituting such pulses in place of audio signals would seriously deteriorate the signal quality. However, by using the audio signal to generate a PCM signal, the latter signal is at least in a form suitable for use in a VTR.
Such PCM signals are generated by sampling the audio signal at a fixed frequency, at least approximately twice as high as the highest audio frequency to be recorded. A multidigit binary pulse signal is generated, the binary numerical value, or code, of which is a function of the amplitude of the signal sample. This pulse signal is the aforementioned PCM signal, and although it is generated as a group of one or more pulses at times determined by the sampling signal, it is usually fed into some type of signal memorizing device. As soon as it is memorized, it ceases to be a signal and becomes a condition of some location in the memory. It can be read out more rapidly than it was read in or less rapidly or at the same speed, and in being read out, the condition, or information, will again become a signal.
U.S. patent application Ser. No. 766,746, filed Feb. 8, 1977, now U.S. Pat. No. 4,141,039, issued Feb. 20, 1979 and U.S. Pat. application Ser. No. 771,350, filed Feb. 23, 1977, now U.S. Pat. No. 4,138,694, issued Feb. 6, 1979, assigned to the assignee of the present application describe in detail VTRs used to record PCM signals based on audio signals. In effect, the pulse signals to be recorded on tape were read out of memory in batches spaced apart by enough time to allow the necessary synchronizing pulses to be inserted in the gaps between successive batches. This required that the pulses in each batch be read out more rapidly than they were read in. In playback, the operation was reversed, and the pulses were reproduced from the information stored on the tape and read into memory at the rate at which they were reproduced. They were then read out in such a way as to return to the original, uniform rate from which they could be reconverted into a high-quality analog audio signal.
In processing signals, even using a high-quality VTR and high-quality tape, there are unavoidable losses of signal information due, for example, to imperfections in the tape and to extraneous noise bursts. Various techniques have been devised in the computer industry to correct errors due to essentially the same causes in computer magnetic tape apparatus. One technique uses an optimal rectangular code (ORC): A. M. Patel and S. J. Hong, "Optimal Rectangular Code for High Density Magnetic Tapes," IBM J. Research Devel., 18 (1974) pp. 579-588. The article by Patel and Hong is described in simpler terms in: N. J. A. Sloane, "A Simple Description of an Error-Correcting Code for High-Density Magnetic Tape," The Bell System Technical Journal, 55 (1976) pp. 157-165. Still another relevant article on error correction, using an interleaving technique is: George C. Clark, Jr. and Robert C. Davis, "Two recent Applications of Error-Correction coding to Communications System design" IEEE Transactions on Communication Technology, Vol. Com-19, No. 5 (1971) pp. 856-863.
Information is recorded on magnetic tape in computer apparatus by encoding or generating the information in binary bit form and applying the bits through multiple heads onto parallel track areas of the magnetic tape. Typically, there are nine parallel tracks across the width of the tape, eight for data and one for recording error-checking parity bits. A succession of such groups of bits recorded in successive areas of the tracks is followed by a group of check bits to form a group codeword. The physical arrangement of tracks and recording areas is basically rectangular. In accordance with the ORC techniques, the check bits are generated in such a way that, together with the parity bits they make it possible to locate and correct errors in a track or, under certain conditions, in more than one track.
However, only one track at a time (except for possible overlapping ends) is recorded in a VTR. Hence the array of related bits is not rectangular but elongated and one bit wide. Each bit is recorded in seriatum, not in parallel with other bits on other tracks.